Marvelously succinct yet detailed. I didn't even suspect that un/follows were part of the blockchain data, I bet that over time they will account or a significant t amount of data as users come, go, or change their preferences or allegiances.
What other minor aspects of user/steemit interaction usually handled server/DB side are committed to the blockchain?
Everything is committed to the blockchain. Think of it as a series of database scripts that you can use to restore a production database from scratch - all that you need to do is "play" the blockchain forward in time and apply each transaction in the blockchain to your database (i.e., if you're building out a database, like http://steemsql.com/)